Tiltrotor aircraft are known in the prior art, including the Bell™ XV-3, XV-15, V-22, and BA609. Tiltrotor and tiltwing aircraft convert between a forward flight cruise mode and a hover mode by changing the orientation of their propellers or rotors and nacelles. Tilting of the nacelle or wing typically occurs about a pivot point commonly called the conversion spindle. The spindle is usually a circular pivot attached to the rotating structure (i.e. wing or nacelle) and inserted into the non-rotating fuselage or wing of the aircraft. For increased redundancy and reliability, the engine driving a rotor on one side of the aircraft is usually configured to have the capability of driving the rotor on the other side of the aircraft by linking the two propulsion systems with what is commonly termed a cross-wing driveshaft. This shaft runs from one propulsion and gearbox system across the wing and into another rotor gearbox and propulsion system. As this driveshaft leaves the wing and enters the nacelle and gearbox of a tilting nacelle it passes through the center of the tilting pivot so that it is not interrupted by the tilting motion. As used herein, a component that rotates can complete an entire revolution about an axis, while a component that tilts can only rotate through a portion of a complete revolution.
FIG. 1 shows a typical prior art tiltrotor aircraft 100 comprising a wing 102 and fuselage 104 with a first tilting rotor system 110 comprising a first rotor blade 112 and first nacelle 118 in aircraft cruise mode corresponding with a generally horizontal position of the nacelle 118. The aircraft is also equipped with a second tilting rotor system 120 on the opposite end of the wing 102. The second rotor system 120 is depicted in conversion from a horizontal position consistent with aircraft cruise mode to a vertical position consistent with helicopter mode. In practice, nacelles 118, 128 on either side of the aircraft in prior art tiltrotors have a substantially identical tilt angle. The tilt angle 136 of a nacelle 128 is the angle 136 between the tilting nacelle axis 138 and the aircraft axis 134. In a typical tilt rotor aircraft 100, the nacelle 104 is also capable of operation in a generally vertical position used in helicopter mode flight. The nacelle 128 tilt angle 136 is usually affected using a tilt actuator and mechanism to convert from helicopter mode flight to aircraft cruise mode. A cross-shaft 106 is disposed within the wing 102 and runs between left and right nacelles 118, 128.
The article “Fail safety aspects of the V-22 pylon conversion actuator” by Duane Hicks published in 1992 summarizes the state of prior art tiltrotor conversion mechanisms. Prior art FIG. 2 is a top view schematic of the Bell™ V-22 tilting system 200 including conversion mechanism, nacelle 218, and wing 202. An engine and gearbox 238 drive a rotor hub 230 coupled to a mast 236 by means of a gimbal 232. A pitchable blade 234 is coupled to the hub 230.
The nacelle 218 and rotor hub 230 pivot as a system about the conversion axis 256. The conversion spindle 250 is aligned and centered on the conversion axis 256. The conversion spindle 250 is supported at two locations, a first inboard bearing 252 carried by the wing 202 and a second outboard bearing 254 also carried by the wing, in order to cantilever the nacelle from the wing. An actuator 240 connected to an actuator spindle 242 aligned with an actuator spindle axis 244 provides motive force to tilt to tilt the nacelle 218 and rotor about the conversion axis 256. The input to the cross-wing driveshaft 260 enters a miter gearbox 262 that converts motion on the miter gearbox axis 264 to the conversion axis 256. The tilting split line 208 is shown as a dashed line.
In the V-22 and other known tiltrotors, the conversion spindle 250 acts as a tunnel between the wing 202 and nacelle 218, through which the cross-wing driveshaft 266 passes. In prior art configurations, the conversion spindle 250 is attached to the nacelle 218 through nacelle structure and a support 254 on the inboard side wall (where inboard is defined as the fuselage side at a parting plane at the rotor rotation axis). This leaves the cross-wing shaft 266 exposed inside the nacelle but outside of the conversion spindle 250. This configuration cantilevers the nacelle 218 on the spindle 250, transferring any bending in the spindle 250 into the nacelle frame at the inboard support 254. A bending load in the spindle 250 can be produced in either forward flight mode or helicopter mode. In forward flight mode, the torque reaction to the rotor and rotor hub 230 induces a bending load on the conversion spindle 250. Lift and drag forces on the nacelle 218 also contribute to this load. In hover, bending is induced in the conversion spindle 250 through the vertical lift generated by the rotor and rotor hub 230 and any lateral thrust vectoring of the rotor thrust.
Prior art tiltrotor aircraft mentioned herein operate with what is termed “gimbaled” or “hinged” rotor systems. That is, their rotors are allowed to tilt about an axis at the hub to nacelle or blade to hub interface, but their masts remain stationary with respect to the non-rotating structure. This hinging means that although the rotors transmit a substantial thrust load, they transmit only small moments from the rotor to the aircraft structure.
A tiltrotor with a hingeless rotor would be able to produce large rotor moments that create operational advantages over traditional tiltrotors. These large moments could easily exceed the moment capability of a traditional conversion spindle. For example, a stiff hingeless rotor such as an Optimum Speed Tilt Rotor (OSTR) as described in U.S. Pat. No. 6,641,365 would provide the increased rotor moment capability of the hingeless rotor and the increased torque output of a large and lightweight rotor. Additionally, it is now appreciated that a wing section outboard of the nacelle (see Tilt Outboard Wing For Tilt Rotor Aircraft, U.S. patent application Ser. No. 11/505,025) can increase the aircraft cruise efficiency but will substantially increase the bending loads through the spindle. These combined loads dramatically increase the applied bending transmitted to the conversion spindle both in hover and airplane flight modes.
The '365 patent and the '025 application, and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
In airborne vehicles, weight is usually critical to the viability of the vehicle. Thus, in designing the attachment structure between the nacelle and wing, designers of the prior art have typically opted for the lowest loading configuration. To this end, the concept of outboard wings on tilt rotor aircraft has been largely ignored due to the high bending moments that applied to the conversion spindle. Conversion spindles in the prior art are short, ending at the first inboard wall of the nacelle and not continuing through to a second interface. In a lightly loaded case, the increased cantilevered load that this configuration transmits to the nacelle is minimal. The implementation of a hingeless rotor vehicle configuration presents benefits and challenges in this area.
Thus, there is still a need for a system that provides (a) a conversion spindle capable of high moment loading in a tiltrotor aircraft, and (b) an integral structural support for an outboard wing, while minimizing weight of the support.